Iowa State University Extension and Outreach Field Agronomists have completed their fall survey of subsoil moisture in northwest Iowa. Subsoil moisture sampling in Iowa dates back to February 1954 when ten sites were chosen for soil moisture sampling. In 1959, the sampling increased to 22 locations, and in 1971, 61 locations were sampled. Today, 18 locations are sampled in northwest Iowa.
The sites were selected for being typical for the major soil type. Each site that was selected has little or no water run-on and no ponding. A 40-feet-by-40-feet area is designated for sampling at the locations, and GPS helps ensure sampling is done at the same location each spring and fall.
The table below compares the results of the last 3 sampling periods.
*Value is estimated to be very close to 0 (wilting point)
The table shows that there was some recharge of soil moisture this fall. We confirm we are going into the winter months with much more moisture in the profile when we compare the fall 2022 and fall 2023 data. Soil moisture is reported in inches of plant available moisture after calculations account for plant available and plant unavailable moisture. A full profile in northwest Iowa can hold about 11 inches of plant available moisture.
Soil moisture is sampled to a depth of five feet in one-foot increments at predetermined locations in the spring and fall. Moisture samples are taken to a depth of 5 feet as corn and soybeans can root to this depth. In years with reduced rainfall, understanding ways to conserve soil moisture is important. Methods of moisture conservation include reducing surface runoff, reducing tillage applications, and increasing ground cover with crop biomass.
The following are the values of plant available moisture for the fall of 2023 collected by ISU Extension and Outreach Field Agronomists.
The drought monitor had significant changes from our sampling period this spring to our most recent sampling this fall. Rainfall received in northwest Iowa over the course of the growing season was variable but allowed some counties to be moved out of the D4 drought category, leaving most in the D0 to D1 categories. Eastern Iowa has increased in drought severity from last spring to this fall.
Subsoil moisture levels across northwest Iowa have improved from last fall, but winter conditions, like amount of precipitation and frost depth, will affect how much moisture is able to infiltrate the soil profile by spring. Subsoil moisture can also be replenished by rainfall when the ground thaws in the spring through the months of March and April. Typical rainfall for those months is three to five inches, and about 80 percent of that rainfall will contribute to subsoil moisture reserves. Corn and soybean crops require about 20 inches of moisture from rainfall and subsoil moisture to produce a crop. That number may increase to 25 inches per season when high temperatures and windy conditions are prevalent during the summer.
Look for updated survey results next spring, or for more information, contact Leah Ten Napel at 712-541-3493 or Gentry Sorenson at 641-430-6715.